There are many circumstances in the oil industry where it is desirable to cut into or through downhole tubular goods within a well. For example, in the course of drilling a well, the drill pipe may become stuck at a downhole location. This may result from "keyseating" or as a result of cuttings which settle within the well around the lower portion of the drill string. In order to remove the drill string from the well, it may be necessary to sever the drill pipe at a location above the stuck point. Similarly, it is often necessary to carry out downhole cutting operations during the completion or operation or abandonment of oil or gas wells. For example, it is sometimes desirable to sever casing or tubing at a downhole location in order to make repairs or withdraw the tubular goods from a well which is being abandoned. In other circumstances, it is desirable to cut slots, grooves or perforations in downhole tubular goods. Thus, it is a common expedient to perforate the casing and surrounding cement sheath of a well in order to provide fluid access to a hydrocarbon bearing formation. Similarly, it is sometimes desirable to perforate tubing in the completion or recompletion of a well.
Chemical cutters can be used to significant advantage in the application of chemicals to cut, sever or perforate downhole tubular goods. For example, U.S. Pat. No. 2,918,125 to Sweetman discloses a downhole chemical cutter which employs cutting fluids that react violently with the object to be cut with the generation of extremely high temperatures sufficient to melt, cut or burn the object. In the Sweetman procedure, halogen fluorides are employed in jet streams impinging on the downhole pipe to sever or perforate the pipe. The attendant reaction is highly exothermic and the pipe is readily penetrated. Examples of chemical cutting agents disclosed in Sweetman are fluorine and the halogen fluorides including such compounds as chlorine trifluoride, chlorine monofluoride, bromine trifluoride, bromine pentafluoride, iodine pentafluoride and iodine heptafluoride. The cutting fluid is expelled from the tool through radial ports formed in the cylindrical wall of the tool in jet cutting streams. In Sweetman, the cutting ports extend radially from a central bore within the discharge head of the cutting tool which terminates in a reduced diameter bore which is open to the lower or front end of the cutting tool. The reduced diameter bore is internally threaded to receive a threaded plug which closes the lower end of the bore. A piston is slidably disposed in the central bore and is equipped with o-rings which bridge the cutting ports when the piston is in the uppermost position. The piston is driven downwardly during the cutting operation. Immediately above the cutting ports is an ignitor section which can include three bodies of steel wool of progressively increasing coarseness and decreasing density toward the discharge end of the ignitor section. The upper portion of the cutting tool is provided with anchoring assembly comprising a plurality of radially projecting bow springs which terminate in downwardly depending slips which are adapted to grip the interior surface of the tubular goods during the cutting operation. The bow spring and slip configurations function to anchor the tool in response to an upward pull applied to the cable supporting the tool.
As further disclosed in U.S. Pat. No. 4,619,318 to Terrell et al., objects may be perforated or in some instances, completely dissolved with no debris left in the well through the use of a downhole chemical cutter. As disclosed in this patent, the chemical cutting tool may be provided with a downwardly extended nozzle provided with a suitable stand-off sleeve. In addition to the halogen fluoride cutting agents as disclosed in the aforementioned patent to Sweetman, further cutting agents as disclosed in the Terrell et al., patent include nitrogen fluoride sources.
Other than the particular adaptation of a nozzle configuration as described in the aforementioned Terrell et al. patent, the normal practice in severing downhole tubular goods is to arrange the cutting ports in the cylindrical wall of the cutting head. This practice is followed in U.S. Pat. No. 4,125,161 to Chammas. Here, the cutting head is a cylindrical member provided with a plurality of discharge ports arranged radially about the outer diameter of the head through which the chemical cutting agent issues in a plane generally perpendicular to the vertical centerline of the head. The cutting ports are bridged with a piston provided with o-rings to prevent the entry of fluids through the ports similarly as with the aforementioned patent to Sweetman. A lower portion of the tool is provided with openings through which well fluid exerts hydrostatic pressure on the bottom of the piston, holding the piston in place before the tool is fired. The Chammas cutting tool incorporates an anchor sub having a plurality of wedges pivoted on an actuating piston near the upper end of the tool in which gas from a propellant charge displaces an actuating piston to cam the wedges outwardly against the tubing string or other object to be cut. The gas from the propellant charge is also employed to force the cutting chemical into contact with a pre-ignitor and thence downwardly through ports in the cutting head and outwardly into contact with the pipe to be severed.
Yet another chemical cutting tool is disclosed in U.S. Pat. No. 4,494,601 to Pratt et al. Here, a lower part of the cutting head structure is open to well fluid and a piston plug is interposed immediately above the cutting ports. The cutting ports may be closed to the exterior of the well by means of an internal sleeve positioned in the bore of the cutting head immediately in front of the piston. When the tool is fired, the fluid pressure developed sets the anchoring means and forces the piston forward, exposing the port to the cutting fluid flowing into the bore from the chemical section. The tool further comprises means in the cutting section in front of the port to receive the piston upon the application of fluid pressure in the tool to lock the piston in place at a location in front of the cutting port. The locking means may take the form of a reduced section in the cutting tool bore which is adapted to receive a portion of the piston in a swedged relationship.
A particularly effective anchoring system for a chemical cutting tool is disclosed in U.S. Pat. No. 4,971,146 to Terrell. In this tool, a chemical module assembly is located intermediate to a propellant assembly and a cutting head assembly. Gas pressure generated by the ignitor of a propellant charge is employed to rapidly move a slip array against a slip expander, during which time the cutting action takes place. After the release of fluid pressure, the slip assembly reliably releases the tool due to the large angle of engagement of the slip segments.